refactor(manifest): split Manifest into ManifestIndex + Manifest single-value type

Manifest now holds exactly one agent and one effective bottle (with
git_user overlay already applied). The old multi-agent/bottle
collection is renamed ManifestIndex. BottleSpec.manifest starts as
ManifestIndex from the CLI and becomes Manifest after _validate()
calls load_for_agent(); all provisioning code downstream reads
spec.manifest.agent / spec.manifest.bottle instead of indexing by name.

bot_bottle/backend/docker/launch.py

@@ -75,7 +75,7 @@ def launch(
     Teardown on exit."""
     stack = ExitStack()
 
-    _bottle_for_revoke = plan.spec.manifest.bottle_for(plan.spec.agent_name)
+    _bottle_for_revoke = plan.spec.manifest.bottle
     _git_gate_dir_for_revoke = git_gate_state_dir(plan.slug)
 
     def teardown() -> None: